Over at Discover, Carl Zimmer tackles a question that many of us are wondering about during flu season. Why do our brains get sluggish when we're sick? It turns out that a neuroscientist named Jonathan Kipnis is working on an answer. He studies how T cells, major players in our immune system, collect in the lining around the brain. He found that mice lacking this T cell buildup have a much harder time learning new tasks than those who do. But how could T cells affect our brains if they just linger in the lining around them? It's possible that they are needed as a barrier between our brains and our immune systems. Writes Zimmer:

When we learn something new, our neurons tear down old connections and build new ones. In the process they cast off lots of molecules. To the immune system, this waste may look like an infection or some other kind of trouble, resulting in inflammation and the release of harsh compounds that normally fight viruses but can also interfere with the brain and its function.

Kipnis suggests that T cells keep this process in check, differentiating between disease and ordinary stress and, when warranted, telling other immune cells to stand down by releasing antagonist molecules that prevent misguided inflammation.

The same T cells that protect the brain from inflammation also work to keep us sharp; and in what appears to be a feedback loop, the mere act of learning reinforces the effect. As mice learn something new, T cells in the meninges produce high levels of a molecule called interleukin 4 (IL-4). IL-4 is an immune system signal that curbs the inflammatory response and, according to research by Kipnis and others, also improves learning. Indeed, when mice lacking the gene for making IL-4 take the water maze test, they do badly, perhaps because their T cells lack a critical signal involved in fast learning.

This theory could explain why we lose our mental edge when we get sick, Kipnis says. When we're healthy, T cells keep the immune cells in the meninges from inflaming the brain. But when we get sick, the T cells loosen their hold to let the immune system attack invading pathogens. The resulting inflammation helps clear out the invaders, but it also blunts learning. When we're sick, Kipnis proposes, it's more important to launch a powerful immune attack than to have a sharp mind. "Everything in life is priorities," he says.